docs: add anti-slop evidence bar to contribution guidelines

Add a strict "evidence bar" section to CONTRIBUTING.md and a PR template
that force every contribution to prove a human exercised the change:
a human-written why, a live before/after evidence block, DCO sign-off,
and a scope statement. Enumerate auto-closed categories (ETHOS edits,
voice/YC cleanup, generated-file-only diffs, blind-AI-sweep PRs). CI
enforcement (changeset-required, DCO check, generated-file guard) is
documented as the next step tied to the Changesets migration rather than
built speculatively.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.github/PULL_REQUEST_TEMPLATE.md

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+<!--
+gstack is AI-coded and proud of it. The bar is EVIDENCE OF REAL USE, not lines
+of code. A PR with no proof behind it gets closed, no matter how clean it looks.
+Fill every section below. See CONTRIBUTING.md → "The evidence bar".
+-->
+
+## Why (in your own words)
+
+<!-- One paragraph: what breaks for a user today, and what this change does about
+it. Not a restatement of the diff. -->
+
+## Live evidence
+
+<!-- REQUIRED. Paste the command(s) you ran and their real output — before and
+after. For a bug: the reproduction, failing then fixed. For a skill change: the
+actual transcript / `claude -p` output. For anything visual: before/after
+screenshots. "bun test passes" alone is not enough — show the behavior you
+changed. -->
+
+```
+# what you ran + what it produced
+```
+
+## Scope
+
+- **Changed:**
+- **Verified live by:**
+- **Did NOT test:**
+
+## Checklist
+
+- [ ] Commits are signed off (`git commit -s`) — DCO
+- [ ] This is not a generated-file-only diff (I edited the source/template and regenerated)
+- [ ] No ETHOS.md edits, and no changes to voice / founder perspective / YC references
+- [ ] New public command / external service / host adapter has an accepted issue linked (or N/A)
+- [ ] Linked issue or reproduction: #